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This macro checks for the size of TYPE using compile checks, not run checks. You can supply extra HEADERS to look into. the check will cycle through 1 2 4 8 16 and any EXTRA_SIZES the user supplies. If a match is found, it will #define SIZEOF_‘TYPE’ to that value. Otherwise it will emit a configure time error indicating the size of the type could not be determined.

The trick is that C will not allow duplicate case labels. While this is valid C code:

  switch (0) case 0: case 1:;

The following is not:

  switch (0) case 0: case 0:;

Thus, the AC_COMPILE_IFELSE will fail if the currently tried size does not match.

Here is an example skeleton script, demonstrating the macro’s usage:

  AC_CHECK_HEADERS(stddef.h unistd.h)
  AC_CHECK_TYPE(ssize_t, int)
  headers='#ifdef HAVE_STDDEF_H
  #include <stddef.h>
  #ifdef HAVE_UNISTD_H
  #include <unistd.h>
  AX_COMPILE_CHECK_SIZEOF(unsigned char *)
  AX_COMPILE_CHECK_SIZEOF(size_t, $headers)
  AX_COMPILE_CHECK_SIZEOF(ssize_t, $headers)
  AX_COMPILE_CHECK_SIZEOF(ptrdiff_t, $headers)
  AX_COMPILE_CHECK_SIZEOF(off_t, $headers)

Source Code

Download the latest version of ax_compile_check_sizeof.m4 or browse the macro’s revision history.


Copyright © 2008 Kaveh Ghazi
Copyright © 2017 Reini Urban

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see <>.

As a special exception, the respective Autoconf Macro’s copyright owner gives unlimited permission to copy, distribute and modify the configure scripts that are the output of Autoconf when processing the Macro. You need not follow the terms of the GNU General Public License when using or distributing such scripts, even though portions of the text of the Macro appear in them. The GNU General Public License (GPL) does govern all other use of the material that constitutes the Autoconf Macro.

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